Additional sagittal correction can be obtained when using an expandable titanium interbody device in lumbar Smith-Peterson osteotomies: a biomechanical study

Background ContextInsertion of intervertebral fusion devices between consecutive Smith-Peterson osteotomies (SPOs) provides an anterior fulcrum during compression, which has been documented to improve achievable Cobb angle correction. Extension of these principles to an expandable device would theoretically provide greater surgical adjustment for flatback and scoliotic cases than a static cage.PurposeTo investigate whether an expandable titanium interbody device would produce greater sagittal correction than a static spacer when used during SPO procedures.Study Design/SettingCadaveric research was performed.Patient SampleSeven T10–S1 human specimens were used.Outcome MeasuresCobb angle changes and range of motion are the physiological measures. No self-report/functional measures were applicable.MethodsBilateral pedicle screws were placed (T11–L5) before Smith-Petersen osteotomy creation from L2 to L4. A transforaminal lumbar interbody fusion titanium expandable implant was placed in each disc space from L2–L3 to L4–L5, which is currently an off-label use of this implant. Initial placement simulated a static spacer, and then incremental device expansion was performed to obtain an intermediate and final height. Lateral fluoroscopic images were taken for Cobb angle evaluation between L2 and L5, and range of motion as observed during application of pure bending moments was captured using a six degree-of-freedom spine simulator. A one-way analysis of variance with Tukey post hoc analysis was performed to determine significant differences (p<.05) between surgical constructs (intact, SPO only, contracted, semiexpanded, and expanded). Study costs were allocated within the research budget of a medical device company, where some authors are salaried employees; another author has been a paid consultant elsewhere. These financial associations were not believed to bias the results.ResultsChange in Cobb angle from L2 to L5 was significantly greater with the interbody spacer compared with SPO alone. Despite an obvious increase in lordosis with expansion height, there were no significant differences between implant expansion states for the L2–L5 Cobb angle. All instrumented constructs were statistically equivalent in every mode of motion once rigid instrumentation was implemented, regardless of expansion state.ConclusionsThe expandable interbody did have a slight effect on lordotic correction; each additional millimeter in height expansion yielded approximately 1° in correction across the three SPO levels. Even without significant differences between the states, an expandable device may allow the surgeon more control of lordotic correction within the operating room than a static spacer alone.